In the past, there has been known a conventional engine control device employing a throttle actuator which is adapted to operate a throttle valve through an electrical signal for controlling the amount of intake air sucked into a vehicular engine. Specifically, the pressure of intake air sucked into the engine is sensed by a pressure sensor, and the width of pulses for driving a fuel injector disposed in an intake passage or manifold is controlled in accordance with the pressure value thus sensed so that the injector is driven every one or two engine revolutions in synchronization with the output signal of an engine rotation sensor which picks up the number of revolutions per minute of the engine. In this manner, the pulse width for the fuel injector is determined to match with the intake air pressure so that a desired amount of fuel is supplied to the engine. Such a control on fuel supply to the engine has been widely used as a speed-density type control and hence a further detailed description thereof will be unnecessary.
On the other hand, the amount of intake air sucked into the engine is controlled by a throttle valve which is disposed in the intake passage and which is in general mechanically opened and closed by an operator through a cable connected between the throttle valve and an accelerator pedal. Recently, however, it was proposed in Japanese Patent Application Laid-Open No. 61-126346 that, instead of directly connecting a throttle valve with an accelerator pedal through a cable, the throttle valve is electrically actuated by an electric actuator, and a portion of such an engine control device has been reduced into practice.
The conventional engine control device described above operates as shown in the flow chart of FIG. 1. Specifically, in Step 10, the output of the accelerator pedal sensor representative of the amount of operation .alpha. of an accelerator pedal imparted by the operator is read out, and in Step 11, the number Ne of revolutions per minute of the engine (hereinafter abbreviated as RPM) sensed by the engine rotation sensor and the pressure Pb of intake air are read out. Then, in Step 12, a target degree .theta. of opening of the throttle valve is calculated based on at least one of .alpha., Ne and Pb thus read out. In general, the target degree .theta. of throttle opening corresponds basically to the amount of accelerator pedal operation .alpha. and is modified or corrected, as necessary, by engine RPM Ne and intake air pressure Pb. For example, in a range in which the engine RPM Ne is low, the rate of change in the amount of intake air changes greatly with slight changes in the throttle opening degree and hence it is rather difficult for the operator to precisely control the amount of intake air to be sucked into the engine by adjusting the amount of operation (depression or return) of the accelerator pedal. To cope with this, it is proposed that in the low RPM range, the rate of change in the opening degree of the throttle valve be made smaller with respect to changes in the amount of accelerator pedal operation .alpha.. On the other hand, it has also been considered that a target value of engine RPM Ne or vehicle speed be set at the operation amount .alpha. of the accelerator pedal so that the actual throttle opening is controlled in a feedback manner based on the differential between the target value and the sensed value of engine RPM Ne or vehicle speed. Further, since the intake pressure Pb is a physical quantity which corresponds to the output torque of the engine, it is possible to improve driving feel by properly adjusting the throttle opening based on the differential between a sensed actual value of intake pressure and a target value thereof which is preset at the operation amount .alpha. of the accelerator pedal. Accordingly, in Step 13, the throttle actuator is driven by an instruction of the control unit to control the throttle valve in such a manner that the actual throttle opening is changed to the target value .theta.. In this case, the throttle actuator may be a pulse-driven open-loop control type actuator such as a stepping motor or a position-feedback control type actuator such as a DC motor.
With the above-described conventional engine control device, when the operation amount of the accelerator pedal increases swiftly, the torque output of the engine increases sharply so that jerk (change in rate of acceleration or deceleration) of the vehicle on which such an engine is installed becomes greater. Accordingly, the vehicle can have excellent acceleration performance on the one hand but riding comfort thereof is impaired on the other hand. This is because reactive force, which develops upon rapid acceleration or deceleration of the vehicle and is transmitted through the engine mounts to the vehicle body due to the general construction of the vehicle, causes the vehicle body to vibrate and at the same time pitching or surging thereof will be induced through the suspension system of the vehicle. In particular, the greater the jerk of the vehicle, the greater discomfort or uneasiness the driver feels.
Furthermore, a rise in the amount of intake air sucked into an engine immediately after acceleration of a vehicle varies depending on the timing of the acceleration, i.e., there is a great difference in the rising rate of the intake air amount depending on whether acceleration is carried out in an early period of an intake stroke or in a late period thereof, as clearly shown in FIG. 2 by the solid line and broken line, respectively. As a result, the magnitude of a jerk of the vehicle developing upon acceleration thereof also varies so that it is difficult to take appropriate measures which effectively suppress jerks of varying magnitude. In addition, when the engine is accelerated or decelerated by opening or closing the throttle valve, the air/fuel ratio varies due to time lags in the fuel control system. To overcome this, various measures are taken to correct or modify the air/fuel ratio and/or ignition timing, but such measures do not provide a uniform effect because of unavoidable variations in the amount of intake air occurring immediately after engine acceleration.